Increase in reach of unrepeatered fiber transmission

ABSTRACT

The present application is directed to techniques and systems for extension of unrepeatered submarine fiber links to provide an increase in reach of unrepeatered fiber transmission. Both single channel unrepeatered systems and multiple channel unrepeatered systems can be used. The multiple channel unrepeatered systems can further employ nonlinearity compensation. The present application is also directed to methods of signal transmission using the unrepeatered systems.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Divisional Application of U.S. patent applicationSer. No. 16/657,776 by Nikola Alic and Vanessa Lynne Karr, entitled“Increase in Reach of Unrepeatered Fiber Transmission,” and filed onOct. 18, 2019, which is Divisional Application of U.S. patentapplication Ser. No. 15/378,869 by Nikola Alic and Vanessa Lynne Karr,entitled “Increase in Reach of Unrepeatered Fiber Transmission,” andfiled on Dec. 14, 2016, which claims the benefit of U.S. ProvisionalPatent Application Ser. No. 62/266,895 by Nikola Alic and Vanessa LynneKarr, entitled “Increase in Reach of Unrepeatered Fiber Transmission,”and filed on Dec. 14, 2015, the contents of which are incorporatedherein by this reference.

TECHNICAL FIELD

This invention is directed to techniques and systems for extension ofunrepeatered submarine fiber links to provide an increase in reach ofunrepeatered fiber transmission.

BACKGROUND

In contrast to amplified and regenerated submarine fiber transmissionfiber, unrepeatered links have no active elements that are deployedin-line. While absence of the amplifier/regenerator element strictlylimits the aggregate reach, such an all-passive link is not subject todominant failure mechanisms that are associated with optical/electricalactive components. Consequently, unrepeatered fiber transmission ishighly reliable and, more importantly, easily serviced: any failure isconfined to the end (launch/receive) points that are not submerged andare easily accessible.

In contrast, the failure in an amplified (regenerated) transmission linkusually originates with degradation of in-line gain/regenerative elementthat cannot be serviced without submersible action and is subject todrastically different economy scales. To address this issue, commercial(repeatered) transmission relies on amplifiers whose performance (gain,bandwidth and noise figure) is traded for higher reliability. Inpractice, this also leads to sub-optimal capacity in all conventional,repeatered fiber links.

Therefore, there is a need for techniques that can provide an increasein reach of unrepeatered fiber transmission in order to increasecapacity and reliability of fiber transmission without the necessity ofemploying active amplifier/regenerator elements.

SUMMARY

The present invention describes techniques and systems for increasingthe reach of unrepeatered fiber transmission.

One aspect of the present invention is a single-channel unrepeateredsystem comprising:

-   -   (1) a transmitter;    -   (2) a transmitting side amplification system;    -   (3) a transmission line devoid of any inline lumped amplifiers;    -   (4) a receiving end amplification system; and    -   (5) a receiver.

In one alternative, the single-channel unrepeatered system operates withabout 100 Watt (50 dBm) launch power. In one alternative, thesingle-channel unrepeatered system operates with a reach of from about750 km to about 790 km.

The single-channel unrepeatered system can employ advanced higher-orderRaman amplification designed to match increased signal launch power; inthis alternative, the system can operate with a reach of at least about1400 km.

Typically, the single-channel unrepeatered system employs an OOK channelof from about 10 Gbps to about 100 Gbps, such as an OOK channel of 10Gbps, 20 Gbps, 30 Gbps, 40 Gbps, 50 Gbps, 60 Gbps, 70 Gbps, 80 Gbps, 90Gbps, or 100 Gbps.

In one alternative, the channel fiber bandwidth is broadened in amultiple of the fiber-native Brillouin bandwidth (20 MHz).

In one alternative, the launched channel is pre-distorted to account forself-phase-modulation (SPM), intra-channel mixing, dispersive broadeningand Raman-induced depletion. Typically, pre-distorting of the launchedchannel is performed by a method selected from the group consisting of:(i) inverting the solution of the nonlinear Schrödinger equation (NLSE)that describes single channel evolution over the unrepeatered link; (ii)implementing analytic approximation of a single channel evolution in adispersive, lossy line; and (iii) choosing a choosing the specificbit-slot waveform shaping with highest stability with respect tononlinear distortion, at given channel rate (speed) and launch power.Alternatively, pre-distorting of the launched channel is performed byselecting the soliton order and subsequently pre-distorting the launchwaveform to minimize nonlinear-dispersive distortion. In anotheralternative, pre-distorting of the launched channel is performed byinverting the solution of the nonlinear Schrödinger equation (NLSE) thatdescribes single channel evolution over the unrepeatered link andwherein inversion of the NLSE occurs at the receiver when the launchedwave comprises an undistorted bit stream (post-compensation) or thecombination of two (pre- and post-distortion).

The single-channel unrepeatered system of claim 1 can employ aparametric frequency comb.

Another aspect of the invention is a multiple-channel unrepeateredsystem comprising:

-   -   (1) a transmitter bank comprising multiple transmitters        corresponding to a multitude of channels that are to be        transmitted over the link generating and optionally        pre-processing the information to be transmitted through the        transmission link;    -   (2) a wavelength division multiplexer that joins the        transmitters and launches them into the transmission link;    -   (3) a transmitting side amplification system;    -   (4) a transmission link devoid of any inline lumped amplifiers;    -   (5) a receiving side amplification system;    -   (6) a wavelength division de-multiplexer used to        disjoin/separate the transmitted optical channels and route them        to their corresponding receivers; and    -   (7) a plurality of receivers each of which is used to receive,        process and detect the information on one or more channels        transmitted to the transmission link.

The information that is to be transmitted through the transmission linkcan be pre-processed.

In one alternative, the plurality of different receivers in step (7) isreplaced or partially replaced by a multiple-channel processingreceiver. The multiple-channel processing receiver can replace some ofthe plurality of receivers or all of the plurality of receivers. Themultiple-channel processing receiver can perform coherent summing todiscriminate noise and increase the received SNR.

The multiple-channel unrepeatered system can employ advancedhigher-order Raman amplification designed to match increased signallaunch power. The multiple-channel unrepeatered system can employ aparametric frequency comb.

Yet another aspect of the invention is a multiple-channel unrepeateredsystem comprising:

-   -   (1) input data to be transmitted through the link;    -   (2) a nonlinearity pre-compensation block computing or        estimating the waveform shapes to be imprinted onto the        transmitted channels leading to the effective partial or        complete cancellation of the nonlinear interaction in        propagation;    -   (3) per wavelength-division multiplex channel computed        waveforms;    -   (4) a bank of waveform generators used to imprint the channels'        waveforms onto the transmitted optical field;    -   (5) a bank of corresponding optical transmitters;    -   (6) a wavelength division multiplexer joining the distinct        transmitters and launching the generated channels into a single        transmission link;    -   (7) a transmitter side optical amplification system;    -   (8) a transmission link devoid of inline lumped amplifiers;    -   (9) a receiver side optical amplification system;    -   (10) a wavelength division de-multiplexer used to separate the        WDM channels and route them to their corresponding receivers;    -   (11) a plurality of receivers each of which being used to        receive, process and detect the information on one or more        channels transmitted to the transmission link;    -   (12) a nonlinearity post-compensating block used for partial or        complete cancellation of the nonlinear interaction in        propagation by processing the received information bearing        waveforms or their sub-sampled versions; and    -   (13) received data obtained after processing away nonlinear        impairment.

In this alternative, the received data can undergo additional processingby at least one technique selected from the group consisting of clockrecovery, carrier phase recovery, frequency offset removal,equalization, and error control decoding.

In this alternative a single channel information can be copied tomultiple carriers that occupy a set of wavelength-nondegeneratepositions, or all transmitted wavelengths within the fiber transmissionwindow.

Nonlinear distortion originating from intra-channel and inter-channelinteraction can be mitigated by a technique selected from the groupconsisting of: (a) inverting the NLSE that describes the interaction ofall channels; (b) inverting the NLSE that describes the interaction ofthe channel subset; and (c) any other predistortion technique thatrelies on knowledge of fiber physical characteristics and channelbit-history.

A subset of the channel information can be copied to multiple carriersoccupying a set of wavelength-nondegenerate positions within the fibertransmission window. When this procedure is employed, typically, theselection of fractional data information that needs to be distributed toa specific wavelength carrier is specifically performed in order to: (i)minimize nonlinear inter-channel interaction; and (ii) minimize thecomplexity of predistortion algorithm used to form the launch waveformor invert the NLSE at the receiver.

In one alternative, the channel information is shared among allwavelength carriers but is encoded by different symbolic or physicalmeans. For example, the channel information can be encoded by differentsymbolic means that include application of different forward-errorcorrection codes (FEC) across the transmission window. As anotherexample, the channel information is encoded by different physical meansthat include encoding distinct carriers by a wavelength-specificmodulation format.

In one alternative, the plurality of different receivers in step (11) isreplaced or partially replaced by a multiple-channel processingreceiver. The multiple-channel processing receiver can replace some ofthe plurality of receivers or all of the plurality of receivers. Themultiple-channel processing receiver can perform coherent summing todiscriminate noise and increase the received SNR.

The multiple-channel unrepeatered system can employ advancedhigher-order Raman amplification designed to match increased signallaunch power. The multiple-channel unrepeatered system can employ aparametric frequency comb.

Another aspect of the invention is a method for transmitting a signal bymeans of a single channel unrepeatered system or a multiple channelunrepeatered system of the present invention as described above suchthat the transmitted signal is clearly received by a recipient in such amanner that the information original present in the signal is receivedby the recipient without significant distortion or loss. Typically, thetransmitted signal is a digital signal representing text, a digitalsignal representing music, a digital signal representing video, adigital signal representing images, or a digital signal representingvoice.

BRIEF DESCRIPTION OF THE DRAWINGS

The following invention will become better understood with reference tothe specification, appended claims, and accompanying drawings, where:

FIG. 1 shows a general single channel unrepeatered system consisting of:(1) transmitter; (2) transmitting side amplification system; (3)transmission link devoid of any inline lumped amplifiers; (4) receivingend amplification system; and (5) receiver.

FIG. 2 shows a multi-channel unrepeatered transmission system consistingof: (1) transmitter bank, i.e. multiple transmitters corresponding to amultitude of channels that are to be transmitted over the linkgenerating and possibly pre-processing the information to be transmittedthrough the transmission link; (2) a wavelength division multiplexerthat joins the transmitters and launches them into the transmissionlink; (3) transmitting side amplification system; (4) transmission linkdevoid of any inline lumped amplifiers; (5) receiving side amplificationsystem; (6) wavelength division de-multiplexer used to disjoin/separatethe transmitted optical channels and route them to their correspondingreceivers; and (7) a plurality of receivers each of which is used toreceive, process and detect the information on one or more channelstransmitted to the transmission link.

FIG. 3 shows a schematic of a multi-channel unrepeatered link withnonlinearity compensation consisting of: (1) input data to betransmitted through the link; (2) a nonlinearity pre-compensation blockcomputing, or estimating the waveform shapes to be imprinted onto thetransmitted channels leading to the effective partial or completecancellation of the nonlinear interaction in propagation; (3) perwavelength-division multiplex channel computed waveforms; (4) a bank ofwaveform generators used to imprint the channels' waveforms onto thetransmitted optical field; (5) a bank of corresponding opticaltransmitters; (6) a wavelength division multiplexer joining the distincttransmitters and launching the generated channels into a singletransmission link; (7) transmitter side optical amplification system;(8) transmission link devoid of inline lumped amplifiers; (9) receiverside optical amplification system; (10) wavelength divisionde-multiplexer used to separate the WDM channels and route them to theircorresponding receivers; (11) a multitude of receivers each of whichbeing used to receive, process and detect the information on one or morechannels transmitted to the optical line; (12) nonlinearitypost-compensating block used for partial or complete cancellation of thenonlinear interaction in propagation by processing the receivedinformation bearing waveforms, or their sub-sampled versions; and (13)received data obtained after processing away the nonlinear impairment(and, possibly additional processing such as, but not limited to clockrecovery, carrier phase recovery, frequency offset removal,equalization, error control decoding).

FIG. 4 shows an example of a parametric frequency comb—an optical sourceemitting a multitude of optical frequencies which can be used as eitherinformation carriers in transmission, or local oscillators in receivers.

DETAILED DESCRIPTION

The present invention describes alternatives for increasing the reach ofunrepeatered fiber transmission without the necessity of introducingactive amplifier/regenerator elements. These alternatives improve thedistance over which signals can be propagated via unrepeatered fibertransmission while maintaining the superior reliability andsignal-to-noise ratio (SNR) that are characteristic of unrepeateredfiber transmission.

The reach of unrepeatered transmission is strictly defined by the launchpower into the fiber that can still result in reliable data recovery atthe receiver node. In practical terms, conventional fiber loss isapproximately 0.22 dB/km, while an advanced receiver equipped withforward-error correction (FEC) operating at 10 GHz has sensitivity ofapproximately −60 dBm. Currently deployed submarine links utilize launchpower of approximately −5 dBm, allowing the reach of 55 dB/0.22 dB/km orabout 250 km. By adding distributed (Raman) and lumped (EDFA) gainelements at both ends of the link and using state-of-the art fiber(˜0.15 dB/km), this reach can be approximately doubled.

To extend the reach further, it is necessary to increase that launchpower and trade it directly against additional link loss. If we assume aconservative launch power density of ˜TW/m², this also means thatstate-of-the-art (SoA), low-loss transmission fiber with the effectivearea of ˜130 μm² can accept ˜100 Watt (50 dBm) launch power. Whencombined with SoA fiber loss (˜0.14 dB/km), this means that unrepeateredreach can be extended to nearly 800 km (such as from 750 km to 790 km),before any distributed amplification is used. With addition of advanced(higher-order) Raman amplification, this reach can be nearly doubled toexceed 1400 km for a 10G bps on-off keying (OOK) channel, before anywavelength-division multiplexing (WDM) is used. By launching multipleWDM channels that can carry different, complementary or identicalinformation, this reach can be extended further.

While having obvious benefits, the increased launch power is prohibitedin SoA links since: (a) Brillouin scattering sets strict limit, wellunder a Watt for continuous-wave (CW) signal and (b) Kerr nonlinearitiesinduces distortions that render received data unrecoverable. Brillouinscattering is associated with the nonlinearity of a medium, inparticular with respect to spatial or temporal variations in refractiveindex. Kerr nonlinearities are associated with variations in refractiveindex induced by an electric field that is due to the light itself beingtransmitted. Both Brillouin scattering and Kerr nonlinearities candegrade data quality.

To address the former, it is necessary to broaden the channel physicalbandwidth in multiples of fiber-native Brillouin bandwidth (20 MHz). Asan example, a 10 Gbps OOK channel provides 10 GHz/20 MHz=500 timesincrease in the Brillouin threshold; a 100 Gbps OOK channel increasesthis power by an additional order of magnitude. However, even if thephysical bandwidth of the channel is increased, either by bit-coding orphase-dithering that does not carry useful information, Kerrnonlinearity will destroy the integrity of the signal. Indeed, thecombined action of wider physical bandwidth and higher optical powerwill lead to channel destruction well before reaching the receiver, evenin case of a single-channel link.

Consequently, to engineer an unrepeatered fiber link with extendedreach, it is necessary to: (1) suppress nonlinear distortion at higherlaunch powers; (2) in some alternatives, use higher-order Raman(distributed) amplifiers designed to match increased signal launchpower; (3) in some alternatives, use a multi-channel processingreceiver.

1. Suppression of Nonlinear Distortion at Higher Launch Powers

For suppression of nonlinear distortion at higher launch powers, a fiberlink operating with a single channel must be treated differently from amultiple-channel (WDM) launch.

a. Single-Channel Nonlinear Cancellation

The launched channel has to be pre-distorted to account forself-phase-modulation (SPM), intra-channel mixing, dispersive broadeningand Raman-induced depletion. The simplest, and not necessarily optimalmethod, is to invert the solution of the nonlinear Schrödinger equation(NLSE) that describes single channel evolution over the unrepeateredlink. Another solution would be to implement analytic approximation of asingle channel evolution in a dispersive, lossy line. Yet anothersolution would be choosing the specific bit-slot waveform shaping withhighest stability with respect to nonlinear distortion, at given channelrate (speed) and launch power. An example (but not complete set) of sucha solution would be to select the soliton order and subsequentlypre-distort the launch waveform to minimize nonlinear-dispersivedistortion. Alternatively, the inversion of NLSE can occur at thereceiver when the launched wave comprises an undistorted bit stream(post-compensation) or the combination of two (pre- andpost-distortion).

b. Multiple-Channel Nonlinear Cancellation

(1) Copying of Channel Information to Multiple Carriers

Channel information is copied to multiple carriers that occupy a set ofwavelength-nondegenerate positions within the fiber transmission window.WDM transmission of high power channels is subject to nonlineardistortion originating from intra-channel and inter-channel interaction.Both can be mitigated by: (a) inverting the NLSE that describes theinteraction of all channels; (b) inverting the NLSE that describes theinteraction of the channel subset; or (c) any other predistortiontechnique that relies on knowledge of fiber physical characteristics andchannel bit-history.

(2) Copying a Subset of Channel Information to Multiple Carriers

A subset of the channel information is copied to multiple carriersoccupying a set of wavelength-nondegenerate positions within the fibertransmission window. The selection of fractional data information thatneeds to be distributed to a specific wavelength carrier is specificallyperformed in order to: (a) minimize nonlinear inter-channel interaction;and (b) minimize the complexity of predistortion algorithm used to formthe launch waveform or invert the NLSE at the receiver.

(3) Sharing Channel Information Among All Wavelength Carriers withDifferent Encoding

In another alternative, the channel information is shared among allwavelength carriers but is encoded by different symbolic or physicalmeans. An example of different symbolic encoding includes application ofdifferent forward-error correction codes (FEC) across the transmissionwindow; an example of different physical means includes encodingdistinct carriers by a wavelength-specific modulation format. Thepurpose of both techniques is to minimize algorithmic expense necessaryto invert nonlinear cancelation and to increase practical launch powerunder which data still can be recovered.

2. Use of Higher-Order Raman Amplifiers

Higher-order Raman (distributed) amplifiers designed to match increasedsignal launch power can be used. In conventional unrepeatered links,higher-order Raman amplification is specifically designed to provideoptical transparency deep into the transmission line. One of thecritical limitations is that Raman pump cannot exceed maximal power. IfRaman amplifier is pumped by excessive power and the signal power is toolow, then created gain results in generation of excess noise thatreduces signal-to-noise ratio (SNR). The detrimental effect of lowlaunch signal power also limits remotely optically pumped amplifier(ROPA). When signal power is too low, the pump power is expended ongeneration of amplified spontaneous emission (ASE), resulting in SNRloss. Consequently, the ability to launch higher optical power alsomeans that both distributed and lumped amplifiers can be engineered tohigher (gain/noise) performance and provide additional link margin withrespect to the conventional link.

3. Use of Multiple-Channel Processing Receiver

In a conventional unrepeatered link, the launch of a single-channel ispreferred over multiple-channel (WDM) link since inter-channel nonlinearinteraction can negate the benefit from replicating or sharinginformation among wavelength-nondegenerate carriers. In contrast, bydenying any nonlinear penalty, multiple channels can be coherentlysummed to discriminate noise, increasing the received SNR. In oneembodiment, if N identical channels are received in such manner, the SNRwill be increased (in linear terms) by N-fold. In another embodiment, ifN wavelength carriers are encoded in order to maximize noisediscrimination by sharing partial data among the carriers, the SNR canbe higher than N-fold.

Accordingly, one aspect of the present invention is a single-channelunrepeatered system comprising:

-   -   (1) a transmitter;    -   (2) a transmitting side amplification system;    -   (3) a transmission line devoid of any inline lumped amplifiers;    -   (4) a receiving end amplification system; and    -   (5) a receiver.

This single-channel unrepeatered system is shown in FIG. 1. In FIG. 1,the system 10 includes the transmitter 12, the transmitting sideamplification system 14, the transmission line 16, the receiving endamplification system 18, and the receiver 20.

In one alternative, this single-channel unrepeatered system operateswith about 100 Watt (50 dBm) launch power.

In one alternative, this single-channel unrepeatered system operateswith a reach of from about 750 km to about 790 km.

In another alternative, this single-channel unrepeatered system employsadvanced higher-order Raman amplification designed to match increasedsignal launch power. In this alternative, the system typically operateswith a reach of at least about 1400 km.

The single-channel unrepeatered system can employ an OOK channel of fromabout 10 Gbps to about 100 Gbps, such as 10 Gpbs, 20 Gbps, 30 Gbps, 40Gbps, 50 Gbps, 60 Gbps, 70 Gbps, 80 Gbps, 90 Gbps, or 100 Gbps.Typically, the channel fiber bandwidth is broadened in a multiple of thefiber-native Brillouin bandwidth (20 MHz).

In one alternative, for a single-channel unrepeatered system, thelaunched channel is pre-distorted to account for self-phase-modulation(SPM), intra-channel mixing, dispersive broadening and Raman-induceddepletion. Pre-distorting of the launched channel can be performed by amethod selected from the group consisting of: (i) inverting the solutionof the nonlinear Schrödinger equation (NLSE) that describes singlechannel evolution over the unrepeatered link; (ii) implementing analyticapproximation of a single channel evolution in a dispersive, lossy line;and (iii) choosing a choosing the specific bit-slot waveform shapingwith highest stability with respect to nonlinear distortion, at givenchannel rate (speed) and launch power. In a preferred alternative,pre-distorting of the launched channel is performed by selecting thesoliton order and subsequently pre-distorting the launch waveform tominimize nonlinear-dispersive distortion. When pre-distorting of thelaunched channel is performed by inverting the solution of the nonlinearSchrödinger equation (NLSE) that describes single channel evolution overthe unrepeatered link, the inversion of NLSE can occur at the receiverwhen the launched wave comprises an undistorted bit stream(post-compensation) or the combination of two (pre- andpost-distortion).

Another aspect of the present invention is a multiple-channelunrepeatered system comprising:

-   -   (1) a transmitter bank comprising multiple transmitters        corresponding to a multitude of channels that are to be        transmitted over the link generating and optionally        pre-processing the information to be transmitted through the        transmission link;    -   (2) a wavelength division multiplexer that joins the        transmitters and launches them into the transmission link;    -   (3) a transmitting side amplification system;    -   (4) a transmission link devoid of any inline lumped amplifiers;    -   (5) a receiving side amplification system;    -   (6) a wavelength division de-multiplexer used to        disjoin/separate the transmitted optical channels and route them        to their corresponding receivers; and    -   (7) a plurality of receivers each of which is used to receive,        process and detect the information on one or more channels        transmitted to the transmission link.

This multiple-channel unrepeatered system is shown in FIG. 2. In FIG. 2,the system 100 includes the transmitter bank 102, the multiplexer 104,the transmitting side amplification system 106, the transmission line108, the receiving side amplification system 110, the de-multiplexer112, and the plurality of receivers 114.

In one alternative, the information that is to be transmitted throughthe transmission link is pre-processed. Pre-processing can be performedby techniques known in the art such as filtering to remove noise orinterference.

Yet another aspect of the present invention is a multiple-channelunrepeatered system employing nonlinearity compensation comprising:

-   -   (1) input data to be transmitted through the link;    -   (2) a nonlinearity pre-compensation block computing or        estimating the waveform shapes to be imprinted onto the        transmitted channels leading to the effective partial or        complete cancellation of the nonlinear interaction in        propagation;    -   (3) per wavelength-division multiplex channel computed        waveforms;    -   (4) a bank of waveform generators used to imprint the channels'        waveforms onto the transmitted optical field;    -   (5) a bank of corresponding optical transmitters;    -   (6) a wavelength division multiplexer joining the distinct        transmitters and launching the generated channels into a single        transmission link;    -   (7) a transmitter side optical amplification system;    -   (8) a transmission link devoid of inline lumped amplifiers;    -   (9) a receiver side optical amplification system;    -   (10) a wavelength division de-multiplexer used to separate the        WDM channels and route them to their corresponding receivers;    -   (11) a plurality of receivers each of which being used to        receive, process and detect the information on one or more        channels transmitted to the transmission link;    -   (12) a nonlinearity post-compensating block used for partial or        complete cancellation of the nonlinear interaction in        propagation by processing the received information bearing        waveforms or their sub-sampled versions; and    -   (13) received data obtained after processing away nonlinear        impairment (and, possibly additional processing such as, but not        limited to clock recovery, carrier phase recovery, frequency        offset removal, equalization, and error control decoding).

This multiple-channel unrepeatered system employing nonlinearitycompensation is shown in FIG. 3. In FIG. 3, the system 200 includes thedata 202, the pre-compensation block 204, the computed waveforms 206,the bank of waveform generators 208, the optical transmitters 210, thewavelength division multiplexer 212, the transmitter side opticalamplification system 214, the transmission link 216, the receiver sideamplification system 218, the de-multiplexer 220, the receivers 222, thepost-compensation block 224, and the received data 226.

In this alternative, the input data and the received data is digitaldata that can be generated by any conventional digital data generationtechnique. In this alternative, the hardware components and connectionscan be hardware components and connections as generally known in theart.

In this alternative, the channel information can be copied to multiplecarriers that occupy a set of wavelength-nondegenerate positions withinthe fiber transmission window.

In this alternative, nonlinear distortion originating from intra-channeland inter-channel interaction can be mitigated by a technique selectedfrom the group consisting of: (a) inverting the NLSE that describes theinteraction of all channels; (b) inverting the NLSE that describes theinteraction of the channel subset; and (c) any other predistortiontechnique that relies on knowledge of fiber physical characteristics andchannel bit-history.

In another alternative of this aspect of the invention, a single channelinformation is copied to multiple carriers that occupy a set ofwavelength-nondegenerate positions, or all transmitted wavelengthswithin the fiber transmission window. Typically, in this alternative,the selection of fractional data information that needs to bedistributed to a specific wavelength carrier is specifically performedin order to: (a) minimize nonlinear inter-channel interaction; and (b)minimize the complexity of predistortion algorithm used to form thelaunch waveform or invert the NLSE at the receiver.

In yet another alternative of this aspect of the invention, the channelinformation is shared among all wavelength carriers but is encoded bydifferent symbolic or physical means. For example, symbolic encoding caninclude application of different forward-error correction codes (FEC)across the transmission window. For example, encoding by differentphysical means can include encoding distinct carriers by awavelength-specific modulation format.

In an alternative embodiment of a multiple-channel unrepeatered system,the plurality of different receivers in Step (7) (for systems notemploying nonlinearity compensation) or in Step (12) (for systemsemploying nonlinearity compensation) can be replaced or partiallyreplaced by a multiple-channel processing receiver. The multiple-channelprocessing receiver can replace some or all of the plurality ofreceivers of those steps. The multiple-channel processing receiver canperform coherent summing to discriminate noise and increase the receivedSNR.

In yet another alternative of a multiple-channel unrepeatered system,either not employing nonlinearity compensation or employing nonlinearitycompensation, a higher order Raman (distributed) amplifier designed tomatch increasing signal launch power is used.

A parametric frequency comb, as shown in FIG. 4, is an optical sourceemitting a multitude of optical frequencies which can be used as eitherinformation carriers in transmission, or local oscillators in receiversin systems according to the present invention.

Yet another aspect of the invention is a method for transmitting asignal by means of a single channel or multiple channel unrepeateredsystem as described above such that the transmitted signal is clearlyreceived by a recipient in such a manner that the information originalpresent in the signal is received by the recipient without significantdistortion or loss.

The signal to be transmitted is any conventionally generated digitalsignal, such as a digital signal representing text, a digital signalrepresenting music, a digital signal representing video, a digitalsignal representing images, or a digital signal representing voice.

Advantages

The present invention provides systems and methods to extend the rangeof unrepeatered fiber transmission without the use of activeamplifier/regenerator elements. This improves the signal-to-noise ratio(SNR) and reliability of such transmission.

The present invention possesses industrial applicability as a system toextend the range of unrepeatered fiber transmission.

The claims of the present invention are directed to specific devices andsystems that are more than general applications of laws of nature andrequire that those practicing the invention employ components or stepsother than those conventionally known in the art, in addition to thespecific applications of laws of nature recited or implied in theclaims, and thus confine the scope of the claims to the specificapplications recited therein. The specific devices and systems of thepresent invention require the use of specific hardware and involvespecific processes involving the hardware that generate a change inphysical state of the hardware and produce physical results.

The inventions illustratively described herein can suitably be practicedin the absence of any element or elements, limitation or limitations,not specifically disclosed herein. Thus, for example, the terms“comprising,” “including,” “containing,” etc. shall be read expansivelyand without limitation. Additionally, the terms and expressions employedherein have been used as terms of description and not of limitation, andthere is no intention in the use of such terms and expressions ofexcluding any equivalents of the future shown and described or anyportion thereof, and it is recognized that various modifications arepossible within the scope of the invention claimed. Thus, it should beunderstood that although the present invention has been specificallydisclosed by preferred embodiments and optional features, modificationand variation of the inventions herein disclosed can be resorted bythose skilled in the art, and that such modifications and variations areconsidered to be within the scope of the inventions disclosed herein.The inventions have been described broadly and generically herein. Eachof the narrower species and subgeneric groupings falling within thescope of the generic disclosure also form part of these inventions. Thisincludes the generic description of each invention with a proviso ornegative limitation removing any subject matter from the genus,regardless of whether or not the excised materials specifically residedtherein.

It is also to be understood that the above description is intended to beillustrative and not restrictive. Many embodiments will be apparent tothose of in the art upon reviewing the above description. The scope ofthe invention should therefore, be determined not with reference to theabove description, but should instead be determined with reference tothe appended claims, along with the full scope of equivalents to whichsuch claims are entitled. The disclosures of all articles andreferences, including patent publications, are incorporated herein byreference.

What is claimed is:
 1. A multiple-channel unrepeatered systemcomprising: (a) a transmitter bank comprising multiple transmitterscorresponding to a multitude of channels that are to be transmitted overthe link generating and optionally pre-processing the information to betransmitted through the transmission link; (b) a wavelength divisionmultiplexer that joins the transmitters and launches them into thetransmission link; (c) a transmitting side amplification system; (d) atransmission link devoid of any inline lumped amplifiers; (e) areceiving side amplification system; (f) a wavelength divisionde-multiplexer used to disjoin/separate the transmitted optical channelsand route them to their corresponding receivers; and (g) a plurality ofreceivers each of which is used to receive, process and detect theinformation on one or more channels transmitted to the transmissionlink.
 2. The multiple-channel unrepeatered system of claim 1, whereinthe information that is to be transmitted through the transmission linkis pre-processed.
 3. The multiple-channel unrepeatered system of claim1, wherein the plurality of different receivers is replaced or partiallyreplaced by a multiple-channel processing receiver.
 4. Themultiple-channel unrepeatered system of claim 3, wherein themultiple-channel processing receiver performs coherent summing todiscriminate noise and increase the received SNR.
 5. Themultiple-channel unrepeatered system of claim 1, wherein the systememploys advanced higher-order Raman amplification designed to matchincreased signal launch power.
 6. The multiple-channel unrepeateredsystem of claim 1, further comprising a parametric frequency comb.
 7. Amethod for transmitting a signal by means of the multiple channelunrepeatered system, the method comprising: receiving a transmittedsignal by a recipient in such a manner that the information originalpresent in the signal is received by the recipient without significantdistortion or loss, wherein the multiple channel unrepeatered systemcomprises: (a) a transmitter bank comprising multiple transmitterscorresponding to a multitude of channels that are to be transmitted overthe link generating and optionally pre-processing the information to betransmitted through the transmission link; (b) a wavelength divisionmultiplexer that joins the transmitters and launches them into thetransmission link; (c) a transmitting side amplification system; (d) atransmission link devoid of any inline lumped amplifiers; (e) areceiving side amplification system; (f) a wavelength divisionde-multiplexer used to disjoin/separate the transmitted optical channelsand route them to their corresponding receivers; and (g) a plurality ofreceivers each of which is used to receive, process and detect theinformation on one or more channels transmitted to the transmissionlink.
 8. The method of claim 7, wherein the transmitted signal is adigital signal representing text, a digital signal representing music, adigital signal representing video, a digital signal representing images,or a digital signal representing voice.
 9. The method of claim 7,wherein the transmitted signal is a digital signal representing text, adigital signal representing music, a digital signal representing video,a digital signal representing images, or a digital signal representingvoice.